The present invention relates to the image and video enhancements domain, especially when the “chroma” component and the “luma” component are not in the same resolution.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.
Most video content received in a TV is encoded in luma and chroma channels, most typically one luma channel Y and two chroma channels Cb and Cr. Most of the time, it is possible to sample the Y channel at full resolution, and to sample the chroma channels at reduced resolution.
Classical chroma sampling formats are:                4:2:2 where the chroma channels are sub sampled horizontally by a factor of two.        4:2:0 where the chroma channels are sub sampled both horizontally and vertically by a factor of 2. The number of chroma samples for each channel Cb or Cr is then 4 times less than in the luma channel.        
Thus there is a need, in such situation, to increase the resolution of the chroma channels to match the resolution of the luma channel, so to produce a chroma sampling format of 4:4:4. This increase in resolution is to be done with the highest possible video quality.
The most common way to address this problem is to simply linearly upscale the chroma channels. Thus, the interpolation filters used for computing chroma samples are fixed (for example Lanczos scaling).
Additionally, some sharpening of the chroma transitions can be done with various methods.
Nevertheless, such methods have drawbacks.
For example, some visual artifact could be detected. A typical artifact is for example visible at the boundary between 2 regions of substantially different colors where usually some “bleeding” occurs because on few pixels, the image has a color different from either of the 2 regions. Other artifacts are a general lack of sharpness or of saturation on sharp contours of colorful contents.
Thus there is a need to improve the chroma sharpness, and to obtain a better image quality while increasing the scale of the chroma components.